Conventionally, lead frames have been used to manufacture a semiconductor device, and the lead frames are generally manufactured by photoetching or pressing a thin metallic plate into a predetermined form. After a semiconductor integrated circuit chip is loaded on a resulting lead frame and the semiconductor chip and an internal lead of the lead frame are interconnected by means of a thin metallic wire, they are sealed with resin or the like and an external lead is formed into a predetermined form to complete a semiconductor device.
When the lead frame is manufactured by photoetching, it is typical to design it so that the widths of the upper and lower surfaces of the external lead, i.e. lengths of the upper and lower sides in cross section of the external lead, are equal. However, when photoetching is conducted by setting the widths of the upper and lower surfaces of the external lead equal, if misalignment occurs between upper surface and lower surface photomasks, then the external lead is made inclined at subsequent external lead formation, making it difficult to mount the semiconductor device on a printed circuit board.
The thinner the external lead is made, the more exacerbated such phenomenon becomes, and in order to help prevent this, as shown in Japanese Patent Application Laid-Open No. 63-81967, there is a technique in which the widths of the upper and lower surfaces of the external lead are set different from each other and photoetching is conducted.
The technique disclosed in Japanese Patent Application Laid-Open No. 63-81967 is described with reference to the drawings. Conventionally, as shown in FIG. 1B, the width X of the upper surface and the width Y of the lower surface are set equal in the cross section of the external lead 3 and, as shown in FIG. 1A, an upper surface photomask 1 and a lower surface photomask 2 have been overlaid on the upper and lower surfaces of a lead frame material 10 to conduct the photoetching. At this time, if there is no misalignment between the upper surface photomask 1 and the lower surface photomask 2, an external lead 3 having a rectangular cross section as shown in FIG. 1B is obtained, and even if the external lead 3 is formed so as to be bent, it is not inclined. However, as shown in FIG. 2A, if misalignment occurs between the upper and lower surface photomasks 1 and 2, an external lead 3 having a cross section of parallelogram form, as shown in FIG. 2B, is obtained, and if the semiconductor device is manufactured using such a lead frame, when the external lead 3 is formed to be bent, the bending moment becomes great in the transverse direction, and as shown in FIG. 3, the external lead 3 is made inclined relative to the predetermined direction and it becomes difficult to mount the semiconductor device on the printed circuit board.
As the measure against this, as shown in FIG. 4B, the widths X and Y of the upper and lower surfaces of the external lead 3 are set different to conduct photoetching. Concretely, as shown in FIG. 4A, during patterning by photoetching, the lower surface photomask 2 for the lead frame material 10 is made wider than the upper surface photomask 1. By so doing, as shown in FIG. 4B, an external lead 3 having a trapezoidal cross section is obtained. If the semiconductor device is manufactured by using a lead frame having the external lead 3 the cross section of which is trapezoidal, then, as compared with the case of the above-mentioned conventional lead frame having the external lead of parallelogram cross section, as shown in FIG. 2B, the bending moment becomes small in the transverse direction when the external lead 3 is formed so as to be bent, and obliqueness of the external lead as shown in FIG. 3 can be prevented. Further, even if, as shown in FIG. 5A, during photoetching, the misalignment between the upper surface and lower surface photomasks 1 and 2 occurs, as shown in FIG. 5B, an external lead 3 having a trapezoidal cross section is obtained, and as compared with the conventional one having a parallelogram cross section, the bending moment becomes small in the transverse direction when the external lead 3 is formed to be bent, and the inclination of the external lead is reduced.
Although Japanese Patent Application Laid-Open No. 63-81967 was heretofore described, according to Japanese Patent Application Laid-Open No. 1-187843, which was disclosed thereafter, the following problems are pointed out. That is, in a semiconductor device having a gull wing type external lead, since the bent portion is formed by bending the external lead to both upper and lower sides thereof, if the cross-sectional forms of each bent portion of the external lead are formed with the same trapezoidal forms, a bent portion whose inside becomes narrower than the outside and another bent portion whose inside becomes wider than the outside result. At the bent portion whose inside becomes wider, stress is concentrated at the inside thereof during bending, and the external lead becomes easy to be cut during the forming process and to be deformed or damaged when the device is selected or carried. Therefore, with reference to FIGS. 6A to 6C, the cross-sectional form of a bent portion 5a of a gull wing type external lead 3 is formed into a trapezoidal form whose lower bottom is shorter than the upper bottom, as shown in FIG. 6B, while the cross-sectional form of the other bent portion 5b is formed into a trapezoidal form whose lower bottom is longer than the upper bottom, as shown in FIG. 6C. Since the bent portions 5a, 5b are thus formed into a trapezoidal cross-section whose upper and lower sides are reversed, the widths of the inside of the bent portions 5a, 5b are formed narrower than those of the outside of the bent portions 5a, 5b.
In the Japanese Patent Application Laid-Open No. 1-187843, it is pointed out that the bent portion in which the inside of the external lead is wider is not preferable because the stress is concentrated to the inside thereof when bending, and as a technique for solving this problem, the width of the inside in the bent portion of the outer lead is arranged so as to become narrower than that of the outside.
Actually, however, there is practically no problem of stress concentration at the bent portion whose inside is wider, and surprisingly, the following problem occurs by making the width of the inside of the external lead bent portion narrower than that of the outside.
(1) As shown in FIG. 12B, if the fulcrum 6 of the bending is present at the narrower side of the external lead 3, since the length which contacts the fulcrum 6 is relatively smaller compared with the case in which the fulcrum 6 is present at the wider side of the external lead 3 as shown in FIG. 12A, the forming of the external lead becomes unstable. In particular, if any misalignment occurs between the upper and lower surface photomasks when the lead frame is manufactured, formation of the external lead becomes more unstable and the external lead is deformed greatly.
(2) As shown in FIG. 6C, since the cross-sectional form of the bent portion 5b shown in FIG. 6A is formed trapezoidal so that the lower bottom is longer than the upper bottom, when the semiconductor device is mounted on the printed circuit board, formation of the solder fillet in the neighborhood of the bent portion 5b becomes insufficient. Since formation of the solder fillet at this portion exerts an extremely great effect on the strength with which the external lead is interconnected to the printed circuit board, the strength is greatly reduced.